^66 THE FACULTY OF HEARING. 



with an inherent potency of development upon given diverging 

 lines, this plastic influence of external surrounding means the 

 convergence of all developing power to a common result^ depending 

 not upon original bias, but on the domination of external influences. 

 To apply this doctrine of '^convergence"" as it is called, to our 

 present case. The tympanal organs of acridia and locusta are 

 obviously not truly homologous though accordant in purpose and 

 function. The morphological resemblance is complete^, yet no 

 true homology can be shown. Nevertheless, observers continue 

 to compare the sense organs of one animal with another, on the 

 assumption that the same morphological development and physical 

 influences operate throughout. But if anatomical evidence does 

 not support such assumptions how is resemblance of structure to 

 be accounted for? The hypothesis of ^"convergence" suggests 

 an explanation of all pseudo-homologies where proof of identity of 

 origin is wanting. ^y "convergence" is understood the 

 occurrence of similar structural forms produced from elements of 

 different origin by plastic influences from without. Matter is 

 supposed to be moulded, not by any inherent tendencies of its own 

 to follow a particular course of evolution (nisus formativusj but 

 by surrounding agencies, so that the whole series of structural 

 phenomena is rendered accordant by the domination of external 

 circumstances over internal endowment of the matter subjected 

 to them. The bias of heredity means, therefore, simply the 

 persistence of unchanged conditions, and is noticed only so long 

 as the same conditions continue ; pseudo-homologies are explained 

 as the result of the same physical surrounding, upon animals of 

 the same asra. A converging series of structural changes thus 

 induced effects its approach towards a common type of structure 

 in the animals subjected to identical modifying influences indepen- 

 dently of their relation to each other (or of the place they may 

 take in our classifications.) But all cases of mimicry where the 

 advantage of the individual lies in its gradual assimilation to 

 another, and which may be explained on the principle of natural 



